The present invention relates to a penetration assembly for sealingly bringing cables such as optical fiber cables through the wall of a containment vessel of a nuclear power plant in a manner so as to prevent gas flow from the containment vessel to the atmosphere.
FIG. 1 shows one presently used type of penetration assembly suitable for installation in the wall of a containment vessel of a nuclear power plant. In the figure, an optical fiber cable 1 is passed through a containment vessel wall (wall not shown). A metal pipe 2 surrounds and physically protects the optical fiber cable 1. A seal material 3 is disposed inside the pipe 2 about the cable 1 for the purpose of creating an air-tight seal. A flange is provided with through holes through which pass the pipe 2, and a seal material 5 fills the space between the flange 4 and the pipe 2 and forms an air-tight seal. Both of the seal materials 3 and 5 usually comprise an epoxy resin or silicone resin.
This type of penetration assembly has a number of problems. The first problem is related to the continuous cycle of heating and cooling to which the penetration assembly is exposed during use. Because there is a great difference in the coefficients of thermal expansion of the metal members (i.e. the pipe 2 and the flange 4) and the resinous seal materials 3 and 5 with which the metal members contact, significant thermal stresses develop in the seal materials 3 and 5. Over a period of time, these repeated thermal sttesses can result in cracks and other unacceptable defects in the seal materials 3 and 5 which destroy their effectiveness.
Another problem with this type of penetration assembly is that the epoxy or silicone resins which make up the seal materials 3 and 5 are subject to degradation by radiation, making them unusable over a long period.